[git] How do I add a submodule to a sub-directory?

For those of you who share my weird fondness of manually editing config files, adding (or modifying) the following would also do the trick.

.git/config (personal config)

[submodule "cookbooks/apt"]
    url = https://github.com/opscode-cookbooks/apt

.gitmodules (committed shared config)

[submodule "cookbooks/apt"]
    path = cookbooks/apt
    url = https://github.com/opscode-cookbooks/apt

See this as well - difference between .gitmodules and specifying submodules in .git/config?